Unless otherwise indicated herein, the materials described in this section are not prior art to the claims and are not admitted to be prior art by inclusion in this section.
A typical cellular wireless system includes a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs), such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices, can operate. In turn, each base station is typically coupled with equipment that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE operating within a coverage area of any base station can engage in air interface communication with the base station and can thereby communicate via the base station with various remote network entities or with other UEs served by the base station.
In general, a cellular wireless system may operate in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to UEs defining a downlink (or forward link) and communications from the UEs to the base stations defining an uplink (or reverse link). Examples of existing air interface protocols include Orthogonal Frequency Division Multiple Access (OFDMA) (e.g., Long Term Evolution (LTE)), CDMA, WiMAX, iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, MMDS, WI-FI, and BLUETOOTH. Each protocol may define its own procedures for initiation of calls, handoff between coverage areas, and functions related to air interface communication.
Depending on the air interface protocol and other factors, each coverage area may be arranged to operate in either a frequency division duplex (FDD) configuration or a time division duplex (TDD) configuration. In an FDD configuration, separate frequencies are used for downlink and uplink communication, so that downlink and uplink communication can occur simultaneously. In a TDD configuration, on the other hand, the same frequency is used for both downlink and uplink communication, and downlink and uplink communications are separated from each other by alternating use of the frequency over time.
The air interface in each coverage area may further have a particular frame structure that defines periodically recurring time units in which information can be communicated between the base station and UEs. For example, a frame structure could define periodically recurring frames of 10 milliseconds, each consisting of a sequence of 10 subframes of 1 millisecond each. Moreover, the example frame structure could then further divide each subframe into a sequence of smaller time units, such as a pair of resource blocks for instance. Other example frame structures are possible as well.
In an FDD system, the downlink and uplink may each separately have such a frame structure, and those frame structures may be synchronized with each other over time. Various control and traffic channels may then be defined for each link. For instance, in each downlink frame, certain subframes or portions thereof might be designated to carry page messages and various system overhead signaling such as a reference signal, and other subframes or portions thereof might be designated to carry bearer traffic from the base station to served UEs. Likewise, in each uplink frame, certain subframes or portions thereof might be designated to carry access messages and various other overhead signaling, and other subframes or portions thereof might be designated to carry bearer traffic from served UEs to the serving base station.
In a TDD system, on the other hand, the air interface may define a single such frame structure for combined downlink/uplink use, with portions of each frame alternating between downlink and uplink in a defined sequence. For example, the first three subframes of each frame may be designated for downlink use, the next two subframes may be designated for uplink use, the next three subframes may be designated for downlink use, and the last two subframes may be designated for uplink use. In each frame of such a system, certain ones of the downlink subframes or portions thereof may again be designated to carry page messages and system overhead signaling such as a reference signal, and other downlink subframes or portions thereof may be designated to carry bearer traffic to served UEs. Further, certain ones of the uplink subframes or portions thereof may be designated to carry access messages and other overhead signaling, and other uplink subframes or portions thereof may be designated to carry bearer traffic to the serving base station.